1. Field of the Invention
This invention generally relates to a polymeric coating composition for coating an implantable device, such as a stent.
2. Description of the Background
Polymeric coatings have been used for coating stents. One of the commercially available polymer coated products is stents manufactured by Boston Scientific. For example, U.S. Pat. Nos. 5,869,127; 6,099,563; 6,179,817; and 6,197,051, assigned to Boston Scientific Corporation, describe various compositions for coating medical devices. These compositions provide to stents described therein an enhanced biocompatibility and may optionally include a bioactive agent. U.S. Pat. No. 6,231,590 to Scimed Life Systems, Inc., describes a coating composition which includes a bioactive agent, a collagenous material, or a collagenous coating optionally containing or coated with other bioactive agents.
The nature of the coating polymers plays an important role in defining the surface properties of a coating. For example, very a low Tg, amorphous coating material induces unacceptable rheological behavior upon mechanical perturbation such as crimping, balloon expansion, etc. On the other hand, a high Tg, or highly crystalline coating material introduces brittle fracture in the high strain areas of the stent pattern.
A current paradigm in biomaterials is the control of protein adsorption on the implant surface. Uncontrolled protein adsorption, leading to mixed layer of partially denatured proteins, is a hallmark of current biomaterials when implanted. Such a surface presents different cell binding sites from adsorbed plasma proteins such as fibrogen and immunoglobulin G. Platelets and inflammatory cells such as monocyte/macrophages and neutrophils adhere to these surfaces. Unfavorable events can be controlled by the use of non-fouling surfaces. These are materials, which absorb little or no protein, primarily due to their hydrophilic surface properties.
Another limitation of current drug eluting stents stems from the fact that the stent is a foreign body. Use of drug eluting stents has proved successful by use of controlled release of anti-proliferative drugs to control restenosis. However, drug eluting stents still have a small, but measurable, incidence of sub-acute thrombosis. Moreover, drug eluting stents have not driven restenosis to zero levels, especially in more challenging patient subsets such as diabetics or patients with small vessels, and/or long, diffuse lesions. A biomaterials-based strategy for further improving the outcome of drug eluting stents is by the use of biobeneficial materials or surfaces in stent coatings. A biobeneficial material is one which enhances the biocompatibility of a device by being non-fouling, hemocompatible, actively non-thrombogenic, or anti-inflammatory, all without depending on the release of a pharmaceutically active agent.
U.S. application No. 2002/0107330 to Pinchuk, et al., describes a composition containing a blend of polystyrene-polyisobutylene-polystyrene copolymer and paclitaxel in various ratios. The composition, when coated onto an implantable device, may be covered with a barrier layer of, or blended with, a polymer or material such as polyethylene oxide or hyaluronic acid (see also Pinchuk, et al., “Polyisobutylene-based Thermoplastic Elastomers for Ultra Long-Term Implant Applications,” Society for Biomaterials, 6th World Biomaterials Congress Transactions, 2000, #1452; Drachman D E, et al., J. Amer. Coll Cardiology, 36(7):2325-2332 (2000); Pinchuk, J. Biomater. Sci. Polymer edn., 6(3):225-267 (1994)). However, it is well known in the art that many biobeneficial materials such as polyethylene oxide or hyaluronic acid are water-soluble and can be leached out of the composition such that the coating may lose biobeneficiality.
The present invention addresses such problems by providing a coating composition for coating implantable devices.